Ichnofabrics and their roles in the modification of petrophysical properties: A case study of the Ordovician Majiagou Formation, northwest Henan Province, China

Abstract

The carbonate rocks of the Ordovician Majiagou Formation from the northwestern part of Henan Province and their diverse trace fossils provide an outcrop analogue for bioturbated, low porosity and low permeability carbonate reservoirs. Three ichnofabrics can be defined based on the ichnotaxonomy, ichnodiversities, individual abundances and overall bioturbation indices. The Balanoglossites ichnofabric represented medium to relatively high-energy sedimentary environments such as intertidal zone and subtidal shoals in the carbonate platform. The Planolites ichnofabric suggested relatively low-energy, protected sedimentary environments such as lagoons in a restricted carbonate platform, but also occurs in the deeper-water depressions in the open carbonate platform. The Chondrites ichnofabric characterized low-energy and reduced oxygen conditions, with burrows penetrating below a shallow redox interface in deeper-water settings that include the distal slope as well as local depressions in the open carbonate platform. Our analysis suggests that the modifications of petrophysical parameters are controlled or influenced by the ichnofabric characteristics (burrow architecture, burrow cross-cutting relationship, degree of bioturbation and burrow connectivity), the permeability contrast between burrow-fills and matrix, and the dolomitization of burrow-fills. The micritic host sediments have negligible porosity and permeability. The bioturbated sediments have significant differences in fabric, texture, porosity and permeability. The burrow-fills are very commonly dolomitised. Each of the burrow-types can enhance reservoir properties, but the greatest differences are associated with the Balanoglossites ichnofabric, in which porosity and permeability can reach over 2% and 70 mD. Preliminary stratigraphic studies indicate that reservoir potential in the subsurface will be linked to an understanding of the distribution of shoal and intertidal facies as well as their associated Balanoglossites ichnofabrics.

Introduction

An ichnofabric is the final record of textural modification produced by the bioturbation or bio-erosion of generations of organisms during their life within or on sediments (Ekdale and Bromley, 1983, Ekdale and Bromley, 1991; Genise et al., 2004; McIlroy, 2008). Ichnofabric analysis has proved to be a useful tool in exploration geology, for example, in estimating palaeobathymetry, assessing sedimentary environments and identifying key stratigraphic surfaces (de Gibert and Martinell, 1998; Pemberton et al., 2000; Zhang and Li, 2000; Buatois et al., 2002; Goldring et al., 2005; Malpas et al., 2005; de Gibert and Goldring, 2007; Shi et al., 2008; Knaust, 2009; Aguirre et al., 2010; Loughlin and Hillier, 2010; Sharafi et al., 2012; Villegas-Martín et al., 2014; Abdel-Fattah et al., 2016; Dorador and Rodríguez-Tovar, 2016). These are all important inputs to the geological models used for determining potential reservoir quality and for modelling basin and petroleum systems (Lin et al., 2010; Qi et al., 2012; La Croix et al., 2012, La Croix et al., 2013; Gingras et al., 2012; Baniak et al., 2013; Gingras et al., 2014; Baniak et al., 2014; Hsieh et al., 2015; Bednarz and McIlroy, 2015; Ben-Awuah and Eswaran, 2015; Rodríguez-Tovar and Dorador, 2015). Recently, geologists have expanded the applications of ichnofabric analysis to address the challenges of production geology (Knaust, 2013, Knaust, 2014; Gingras et al., 2014; Eltom et al., 2019), that is, assessing the ways in which bio-induced porosity and permeability can be exploited to enhance hydrocarbon recovery (Gingras et al., 2004a; Pemberton and Gingras, 2005; Cunningham et al., 2006; Pemberton et al., 2008; Gingras et al., 2009; Gordon et al., 2010; Cunningham et al., 2012; La Croix et al., 2012, La Croix et al., 2013; Baniak et al., 2013, Baniak et al., 2014; Rashid et al., 2015; Golab et al., 2017a, Golab et al., 2017b; Knaust et al., 2020). At present, ichnofabric research is at least an important part of ichnology and sedimentology fields. The International Congress on Ichnology has carried out numerous widespread academic communications on ichnofabrics (Zhang et al., 2015; Fan and Gong, 2017; Niu et al., 2019).

The Ordovician strata in the North China Craton have been extensively studied (Ye, 1983; Wang et al., 1989; Feng et al., 1990; Qi et al., 1997; Qi and Shi, 2002; Hu et al., 2011; Niu et al., 2013, Niu et al., 2015; Dong and Niu, 2015a, Dong and Niu, 2015b). The preliminary and large-scale sedimentary environment and lithofacies palaeogeography of the Ordovician of the North China Craton were established by Ye (1983), Wang et al. (1989) and Feng et al. (1990). Qi et al. (1997) and Qi and Shi (2002) identified some trace fossils of the northwest Henan province and used them to identify sequence boundaries and build a sequence stratigraphic framework (Qi et al., 1997; Qi and Shi, 2002). Hu et al. (2011) and Niu et al. (2015) provided a systematic identification and classification using trace fossils of the Ordovician carbonates in the study area and established a depositional model based on combined ichnology and sedimentology. Dong and Niu, 2015a, Dong and Niu, 2015b studied the reservoir potential and pore evolution of burrows of the third Member of the Ordovician Majiagou Formation (Dong and Niu, 2015a, Dong and Niu, 2015b).

This paper summarizes the ichnofabrics and their distribution throughout the Majiagou Formation. We undertook detailed observations of textures and systematically measured the porosity and permeability of the carbonate rocks to obtain a better understanding of the potential reservoir properties of the formation. It is also significant to understand some local aquifers formed by bioturbation in the Majiagou Formation and the groundwater may flow into the nearby coal mines during the course of exploiting the Carboniferous-Permian coal measures. This study may provide applications to the characterization of the other Lower Palaeozoic tight bioturbated carbonate reservoirs such as those in the Tarim, Erdos and Sichuan basins, basins in western Canada, and the Williston Basin of North America.